1. Make certain that ALL your finances are completely separated. And things are fairly paid for. Theres four of you, so you only pay for 1/4 of your housing (unless it belongs to husband then you pay nothing) and food you only buy food for yourself. Contribute a nominal amount to cover your utilities, etc. Lock up your personal stuff and shampoo, toothpaste, etc. 

Disengage from them all. Husband and his mini-wife moochers. Do nothing for them (* see above)

2. Get your ducks in a row. To leave. You arent married. If you hold joint assetts, get a lawyer involved.

Talking to him and he goes into rage mode? Oh yea that happens to me, and its a deflection tool, to get you to shut up.

DH needs to set an end date of when these girls will be out on their own. He needs to have the talk with them and then enforce the move out, even if that means putting their stuff on the curb. 

Do the SD's work? Go to college? 

Once they are out, DH needs to let them know they will not be moving in under any circumstances. 

My SD's are similar, and I've been with their dad 13 years also. Their ages are 18, 20, and 22 and all three live with BM because she has no rules. My DH agreed they will never move back with us and he has started to stand up for himself more and more, thank god. I hope this works out for you but you do have to tell DH that you won't tolerate any more, and mean it. 

It’s been my experience that a ‘Disney Dad’ will be reluctant to jettison his adult spawn, despite the fact that they cause disruption in the home and discomfort for the DD’s wife/partner. These men are so fraught with guilt over having created a broken home for their children (and, obviously, because they love them and view them through rose-coloured glasses), that they become incapable of finding a balance of between what's acceptable for a healthy marriage and what’s best for their adult bios.

What worked for my particular, domestic horror (manipulative mini-wife) was consulting with a therapist who specialized in step-family crises. Like any person who is indoctrinated into a particular mindset, after years of habituation, Disney Dads need a neutral, third party to open their minds to reality.

Not long after I moved in with my husband, we began to have hurtful disagreements that we’d never experienced during the 5 years of our courtship. It was so awful! I’d left my charming home, my family, the job that I loved and all of my friends to move far away and marry my husband. I felt that I'd been plunged from a happy, independent life into a domestic hell, ruled by a furtive, devious, teenaged manipulator. 

My husband is an intelligent and reasonable man who loves me deeply and was prepared to do whatever it took to save our marriage. Our therapist convinced him that he was damaging his daughter with his infantilizing (the therapist found SD to be seriously immature for a 13-year-old) and his failure to correct the girl’s passive/aggressive behaviours. The therapist also impressed upon my husband that I’d had enough and was ready to walk. Neither of us wanted to end the marriage; we adored each other!

Without the eye-opening wisdom/experience of our talented counsellor, I doubt that our marriage could have survived. I was a confident woman, who had enjoyed more than one offer from attractive, successful men, and was certain that I could land on my feet in the event that I dissolved the marriage. In my opinion, it’s important that your husband appreciate the fact that there are limits to your tolerance (right, Lilly?) and that you will leave if he fails to address his domestic issues.

Life is short, Arielle, and the longer you spend in an unjust, troubled environment, the more you cheat yourself of the happiness that you deserve.

What worked for my particular, domestic horror (manipulative mini-wife) was consulting with a therapist who specialized in step-family crises. 
 

What granny's said, all of it. And especially this.  If DH and I had not seen a qualified therapist familiar with step-family garbage, we would not be together today. That objective third party observance and input is gold. 
Hiding in your own house is nightmarish. If SO doesn't agree to see a counselor with you, would you go alone? 

They are adults. Have your attorney send them an eviction notice. When daddy loses his shit, tell him that this is not negotiable.

Why should you leave. They can. Then the choice for daddy is to stay, or go.  Based on his choice, respond accordingly.

Start banking assets now, just in case.

First, you should make sure that you are in a position to stand on your own two feet.  could you financially make it without him?  is the home his?  does he provide the majority of the finances?  do you work and if so, could you make it?  Because having the ability to walk away makes things a  lot easier for you... otherwise.. he has you in a tough spot and it will be harder to talk to him about his daughters should be financially independent.. if you aren't able to do that yourself.

Second, the approach here can make a difference.  Doesn't he want his girls to be happy and have a fulfilling independent life and enjoy living on their own?  Why doesn't he want to help them do that.. encourage education...jobs.. etc.. (I am assuming for this that when you say that they are 8 year olds.. that you are just meaning immature and that they are not actually disabled in some way mentally or medically).  Parents should want to see their kids blossom.. build enriching lives.. why doesn't he want that for his daughters?  

See the focus is wanting the best for them.. doesn't he want the best for his kids?  How can he get them to reach for their best life?  

This unhealthy relationship with your SO and his DD is already set.  It's the three losers together against you.  If your SO rather live with his DD  then you. It's unfortunate,but he must go..  then all of them must get a job. To pay for a appointment, food, electricity. Ect.  This will sound frightening to him